In recent years, optical disc playback apparatuses have made a remarkable increase in recording/playback speed. The optical disc playback apparatuses have achieved the increase in the playback speed by increasing the rotation speed of optical discs.
However, when the rotation speed of an optical disc is increased, vibration due to an eccentricity of the optical disc adversely affects control such as servo, resulting in a problem that the user of the optical disc playback apparatus might feel uncomfortable. In view of such a problem, when an optical disc having a large eccentricity is loaded on the optical disc playback apparatus, the optical disc playback apparatus limits the rotation speed of the disc to prevent the adverse effect of vibration due to the disc having the large eccentricity. Therefore, the measurement of vibration amplitudes is an important technique for preventing the adverse effect of vibration due to the disc having the large eccentricity in the optical disc playback apparatus.
FIG. 8 is a block diagram illustrating a conventional optical disc playback apparatus 800 having a structure for detecting vibration with an acceleration sensor. In FIG. 8, reference numeral 801 denotes a base, reference numeral 802 denotes a disc motor mounted on the base 801, reference numeral 803 denotes an insulator supporting the base 801, reference numeral 804 denotes a disc mounted on the disc motor 802, reference numeral 805 denotes an acceleration sensor mounted on the base 801, and reference numeral 806 denotes a measurement unit for measuring the amount of eccentricity on the basis of the output of the acceleration sensor 805.
Hereinafter, the operation of the conventional optical disc playback apparatus 800 will be described. When the disc motor 802 on which the disc 804 is mounted is rotated at a predetermined rpm (revolutions per minute), a centrifugal force in proportion to the amount of eccentricity of the disc 804 is generated. The base 801 supported by the insulator 803 vibrates at an amplitude that is determined by the amount of eccentricity of the disc 804, the total mass of the base 801 and the constituents mounted on the base 801, and the spring constant of the insulator 803.
The vibration of the base 801 is converted into an electric signal by the acceleration sensor 805 mounted on the base 801. The measurement unit 806 measures the vibration amplitude of the base 801 on the basis of the electric signal obtained by the acceleration sensor 805.
Then, the measured vibration amplitude is compared with a preset threshold value, thereby determining a maximum rotation speed of the disc mounted on the optical disc device.
Furthermore, FIG. 9 is a block diagram illustrating a conventional optical disc playback apparatus 900 performing vibration detection by using track counting, which is disclosed in Japanese Published Patent Application No. 2000-113581. In FIG. 9, the same reference numerals as those shown in FIG. 8 denote the same or corresponding parts, and reference numeral 901 denotes an optical head which is suspended over the base 801 via an elastic member 902.
Reference numeral 903 denotes a light beam applied to the disc 804 from the optical head 901, and reference numeral 904 denotes information recording tracks which are concentrically or spirally formed on an information recording surface 804A of the disc 804 at a constant pitch. Furthermore, reference numeral 905 denotes a track cross detection unit for generating track cross pulses and a cross direction signal from signals which are reproduced when the light beam 903 crosses the information recording tracks 904; reference numeral 906 denotes a count unit for counting the track cross pulses; reference numeral 907 denotes a measurement unit for determining the amount of eccentricity from the counting result of the count unit 906; and reference numeral 908 denotes a motor control unit for controlling the rpm of the disc motor 802, and outputting rotation angle information to the measurement unit 907.
The distance between the optical head 901 and the disc 804 is kept constant so that the focus of the light beam 903 is positioned on the information recording surface 804A of the disc 804. The position of the optical head 901 relative to the disc 804 in the direction of the radius of the disc 804 (the direction indicated by an arrow R) has vibration characteristics represented by a natural frequency of vibration foA which is determined by the spring constant of the elastic member 902 comprising a material such as metal, resin, or rubber, and the mass of the optical head 901.
The base 801 is supported by the insulator 803 comprising a material such as metal, resin, or rubber. When the centrifugal force generated by the rotation of the disc 804 is propagated through the disc motor 802 to the base 801, the base 801 vibrates on the basis of vibration characteristics represented by a natural frequency of vibration foM which is determined by the total mass of the constituents including the base 801, the optical head 901 mounted on the base 801, the disc motor 802, and the disc 804, and the spring constant of the insulator 803.
The motor control unit 908 rotates the disc motor 802 at a first rpm that is sufficiently lower than the natural frequency of vibration foA. The optical disc 804 mounted on the disc motor 802 is rotated at the first rpm.
Within the range of the first rpm that is sufficiently lower than the natural frequency of vibration foA, the optical head 901 vibrates together with the base 801. The relative position between the optical head 901 and the optical disc 804 hardly changes. Therefore, at the first rpm that is sufficiently lower than the natural frequency of vibration foA, the light beam 903 crosses a number of information recording tracks 904 equivalent to the amount of eccentricity of the information recording tracks 904. The light beam 903 generates as many track crosses as the information recording tracks 904 that the light beam 903 has crossed.
Then, the track cross detection unit 905 detects the track crosses corresponding to the information recording tracks 904 the light beam 903 has crossed, on the basis of the playback signals of the optical head 901, and generates track cross pulses corresponding to the detected track crosses. The track cross detection unit 905 outputs the generated track cross pulses to the count unit 906.
The count unit 906 counts the track cross pulses for one rotation of the disc 804 on the basis of the rotation angle information from the motor control unit 908. The measurement unit 907 stores a count result N1 of the track cross pulses for one rotation of the disc 804 which are counted by the count unit 906.
Next, the motor control unit 908 rotates the disc motor 802 at a second rpm that is higher than the natural frequency of vibration foA and lower than the natural frequency of vibration foM. Then, a centrifugal force is generated in the disc 804 due to the eccentricity of the disc 804. The base 801 vibrates at an amplitude that is determined by the amount of eccentricity of the disc 804, the total mass of the base 801 and the components mounted on the base 801, and the spring constant of the insulator 803.
When the disc motor 802 rotates at the second rpm that is higher than the natural frequency of vibration foA and lower than the natural frequency of vibration foM, only the base 801, disc motor 802, and disc 804 vibrate together while the optical head 901 is in the static state. Therefore, the relative displacement between the disc 804 and the optical head 901 becomes equal to the vibration displacement of the base 801. As a result, the light beam 903 generates as many track crosses as the tracks equivalent to an amount in which the vibration amplitude of the base 801 is added to the amount of eccentricity of the information recording tracks 904.
The track cross detection unit 905 detects the track crosses which number as many as the tracks equivalent to the amount comprising the amount of eccentricity of the information recording tracks 904 and the vibration amplitude of the base 801, and generates as many track cross pulses as the tracks equivalent to the amount comprising the amount of eccentricity of the information recording tracks 904 and the vibration amplitude of the base 801. The track cross detection unit 905 outputs the generated track cross pulses to the count unit 906.
The count unit 906 counts the track cross pulses for one rotation of the disc 804, on the basis of the rotation angle information from the motor control unit 908. The measurement unit 907 subtracts the count result N1 from a count result N2 obtained by the count unit 906, thereby obtaining the vibration amplitude of the base 801 alone.
Then, the measured vibration amplitude is compared with a threshold value which is prepared for each disc type, thereby determining a maximum rotation speed of the disc loaded on the optical disc device.
Furthermore, when detecting a vibration amplitude by a track counting method, setting of a track count filter according to the vibration speed becomes important for preventing a false count. Although a method of setting a track count filter for vibration detection is not general at present, a method disclosed in Japanese Published Patent Application No. Hei. 8-45089 is known as a method of setting a track count filter at seeking. To be specific, an optical disc device disclosed is provided with a PLL circuit for outputting a control frequency according to the moving speed of an optical head when the optical head is moved to a target track position on the surface of an optical disc by a thread motor; an F/V conversion circuit for converting the control frequency into a voltage; and a ripple detection filter for attenuating a noise component included in a ripple signal from an envelope detection circuit, whose filter characteristic is changed according to the voltage from the F/V conversion circuit. Thereby, the cut-off frequency of the filter is varied according to the moving speed of the optical head.
In the above-mentioned conventional method for detecting vibration by using the acceleration sensor 805, since the vibration itself of the disc device is measured by using the acceleration sensor, a maximum rotation speed can be determined by comparing the measurement result with a single threshold value that is independent of the disc type. However, an increase in cost due to mounting of the acceleration sensor 805 and a signal amplifier for amplifying the signal from the acceleration sensor 805 is considerable. Furthermore, it is necessary to secure a space for mounting the acceleration sensor 805 and the signal amplifier.
Furthermore, in the conventional method for detecting vibration using track counting, which is disclosed in Japanese Published Patent Application No. 2000-113581, the measurement of vibration amplitudes can be carried out with reduced cost and space, without using an acceleration sensor. However, since track counting is employed, if the track pitch of the disc varies, the value of the measured vibration amplitude also varies even under the same vibration condition, and therefore, a threshold value must be prepared for each disc having different track pitches, and a maximum rotation speed is determined by comparing each threshold value. However, the vibration of the disc device varies depending on the setting condition of the disc device on a system, and moreover, the vibration amplitude with which the user feels uncomfortable depends on the human sense and so it varies depend on the user. Therefore, a threshold value must be set for each system on which the disc device is to be mounted. However, since threshold setting varies depending on the loading condition or the like of the disc to be used on the disc device, a large amount of data must be measured, and the amount of data further increases according to the number of disc types, whereby an enormous amount of time is required to measure such data.
Furthermore, in the method of setting a track count filter disclosed in Japanese Published Patent Application No. Hei. 8-45089, when performing a traverse seek, how the seek speed will change is known in advance because acceleration or deceleration of the seek speed is carried out according to a speed profile that has previously been determined, and the response speed to the change in the filter characteristics can be appropriately set according to the predetermined profile. However, in the case where the frequency and the speed change in the frequency are changed considerably due to the eccentricity of the disc or the vibration amplitude like the track count pulses for vibration detection, it is difficult to set the response speed to the change in the filter characteristics, resulting in false count when the response speed is too high or too low.
The present invention is made to solve the above-described problems. Therefore, an object of the present invention is to provide an information disc recording/playback apparatus and a vibration detection method for the information disc recording/playback apparatus, which can measure vibration amplitudes with reduced cost and space without using an acceleration sensor, perform vibration detection by using a single threshold value that is independent of the disc type, and perform accurate vibration detection even when direction detection is difficult.
Another object of the present invention is to provide an information disc recording/playback apparatus and a vibration detection method for the information disc recording/playback apparatus, which can provide a method of controlling the cut-off frequency of a noise removal filter for track counting that is most suited to vibration detection, and perform accurate vibration detection with less of a false count.